While deuterium H-2, an isotope twice as heavy as hydrogen, is predominantly used in nutrition research, nitrogen-15 is the most common stable isotope used in agriculture. Many other stable isotopes ...

and the sum of the protons and neutrons gives the atomic mass, the number of neutrons defines the isotope of that element. For example, most carbon (≈ 99 %) has 6 protons and 6 neutrons and is written ...